1. Field of the Invention
This invention relates to a pressure vessel used under high operating pressures of 200 to 300 kg/cm.sup.2 G, such as an extraction vessel which constitutes the main equipment of an apparatus for executing supercritical extraction.
2. Description of the Prior Art
The applicant of the present invention previously invented a pressure vessel and filed a patent application for it (Japanese Patent Unexamined Publication No. 63-158396, hereinafter referred to as "prior art invention"). As shown in FIGS. 4 and 5, the pressure vessel of this prior art invention comprises a cylindrical member a constituting the housing of the vessel, and two covers b respectively fitted into openings provided at both ends of the cylindrical member a, the two covers b being connected to each other through the intermediary of tie rods c to form an integrated unit which is slidable within the cylindrical member a.
As shown in FIG. 5, the integrated unit of the above-described prior art invention is formed by connecting the two covers b by means of three tie rods c. However, this results in a cross section which is asymmetrical in the vertical direction. Consequently, calculation of the structural strength of a high-pressure vessel, which is an indispensable aspect of the design thereof, is inevitably rather complicated.
If the tie rods c are connected to the covers b by means of screw sections d, as shown in FIG. 7, the screw sections d must be designed to be large. This, however, affects the strength. To cope with this, it is necessary for the overall structure to have a considerable size. This, in turn, results in the thickness t of the whole being rather too large.
If, as shown in FIG. 8, large screw holes e are provided in the covers b and the covers b and the tie rods c are connected to each other by means of nuts f, the engagement sections will influence the strength, so that thickness t of the covers b has to be large. Moreover, it is not easy for gaskets g alone to effectively seal the screw sections.
It might be considered possible to connect the tie rods c to the covers b by means of welding, as shown in FIG. 9. However, it is not possible for the welded sections h alone to withstand the great load imposed on them by the high pressure involved.